Various methods for electrically connecting packaged and unpackaged integrated circuits to printed circuit substrates are well known in the art. On such method uses flip-chips. A flip-chip is an unpackaged integrated circuit device whose bonding pads bear solder bumps which are substantially hemispherical-shaped. The solder bumps typically are made of a tin-lead composition.
Generally, one or more flip-chips are electrically connected to a printed circuit substrate along with other electrical devices to form an electrical circuit. One or both sides of a printed circuit substrate can be used as an active side, which has an interconnection pattern of electrically conductive material (e.g. copper) in which to interconnect the various devices used in the circuit. For the purposes of flip-chip attachment, the substrates interconnection pattern has a number of contact pads defined thereon which correspond to the location and orientation of the solder bumps on a particular flip-chip which may be attached to the printed circuit substrate. One method of connection is controlled collapse chip connection (C4) soldering which involves aligning the bumps of the flip-chip with corresponding wettable solder contacts of the substrate, and then heating the solder to induce reflow and electrical connection between the chip and substrate.
FIG. 1 shows a cross-sectional view of a typical prior art flexible printed circuit substrate 10. Substrate 10 comprises an organic epoxy core 12 and a plurality of conductive layers 14 separated by insulation layers 16. The substrate has a plurality of contact pads 18 for electrically coupling the substrate to other electronic devices, such as flip-chips. Tin-lead solder bumps 20 are generally formed on contact pads 18 to facilitate the attachment of an electrical device, such as a flip-chip, to the printed circuit board. The outermost layer of substrate 10 typically includes a permanent photoresist layer 24.
The contact pads 18, as depicted in FIG. 1, are generally known as pad-on-off via contacts since the contacts do not themselves reside in the substrate vias 22. As shown in FIG. 1, the contacts pads comprise a conductive layer 26 that connects the contacts to the substrate vias 22. Until recently, only pad-off-via have been used on flexible, epoxy printed circuit boards, mainly because of manufacturing limitations. However, the number of I/O's (inputs/outputs) on flip-chip devices has risen to a point where the exclusive use of pad-off-via contacts is impractical in some instances. The need to maximize usable substrate real estate has led to advanced manufacturing techniques that now permit the manufacture of printed circuit substrates containing both pad-off-via and pad-on-via contacts.
It is appreciated that the deposition and reflow of solder on a printed circuit substrate must be precisely controlled to control the height of the solder bumps after soldering. As is well known in the art, controlling the height of solder bumps after reflow is necessary in order to prevent the surface tension of the molten solder bumps from drawing the flip-chip excessively close to the substrate during the reflow process. Moreover, it is appreciated that controlling the height and volume of solder bumps is necessary to produce reliable electrical interconnections.
FIG. 2 illustrates a printed circuit substrate 40 containing both a pad-on-via and a pad-off-via contact 42 and 44, respectively. Since the depths of the pad-off-via and pad-on-via contacts differ, those problems associated with controlling the height and volume of the printed circuit substrate solder bumps are accentuated.
What is needed then is a low cost method for forming reliable connections between surface mounted electronic components and substrates containing both pad-off and pad-on via contacts. As will be seen, the present invention provides a method for controlling the height and volume of solder bumps on printed circuit boards containing pad-off and pad-on via contacts.
The invention provides a printed circuit substrate comprising a plurality of contact pads and solder on the contact pads. The contact pads comprise at least one pad-on-via and at least one pad-off-via. The solder has a first volume on the pad-on-via and a second volume on the pad-off-via. The volumes have surfaces, facing away from the respective vias, which are at least partially co-planar and at least partially electrically unattached.